Rotary displacement compressor with liquid circulation system

ABSTRACT

A rotary displacement compressor having an inlet channel for introducing low pressure gas, an outlet channel through which compressed gas escapes, and at least one rotor mounted in bearings and operating in a working space. The compressor includes a liquid injector for injecting liquid into the working space, a liquid separator provided in the outlet channel for separating liquid from the compressed gas, and a pressure liquid conduit connecting the liquid separator and the liquid injection port. A bearing lubrication circuit includes a tank, a pump, a supply conduit for supplying liquid from the pump to the bearings in which the rotor is mounted, a withdrawal conduit for withdrawing liquid from the bearings to the tank, and a pump inlet conduit for supplying liquid from the tank to the pump. The bearing lubrication circuit further includes a leakage path connecting the tank to a first cavity in the working space and a drainage connection connecting the tank to a second cavity in the working space, the first cavity having a higher pressure than the second cavity.

The present invention relates to a rotary displacement compressor havinga simple and reliable dual liquid; circulation system.

BACKGROUND OF THE INVENTION

Injection of liquid, usually oil for lubrication, sealing and coolingpurposes, into the working space of a compressor of this kind is widelyused, in particular in screw compressors. The use of separate circuitsfor this oil injection and for the bearing lubrication system isadvantageous when the compressor operates at high pressures and with aworking medium that can dissolve in the oil. The higher the pressure ofthe oil is, the more the working medium can dissolve in the oil. Theworking medium dissolved in the oil will decrease the viscosity of theoil and therewith its lubricating ability. The provision of a separatecircuit for the bearing lubrication makes it possible to use a lowerpressure for the bearing lubrication, than when taking the oil from theoil separator, where compressor outlet pressure prevails. Therethroughthe above mentioned problem is avoided. Such systems are earlier knowne.g. from GB-A-2 008 684 and U.S. Pat. No. 4,394,113.

GB-A-2 008 684 discloses an oil injected rotary screw compressor inwhich a portion of the oil in the oil separator is throttled andconducted to a second oil separator at a lower pressure. From thissecond separator the oil is pumped to the bearings from where it isdrained back to the pump. This arrangement requires an extraoil-separator and a recirculation circuit for the working medium in thatseparator, and therefore becomes somewhat circumstantial.

U.S. Pat. No. 4 394 113 discloses a rotary screw compressor having amain oil circulation circuit including an oil separator and a secondaryoil circulation circuit including an oil tank, which secondary circuitoperates at a lower pressure and provides lubrication of the bearings.Since oil might leak from one of the circuits to the other, means areprovided for maintaining an appropriate amount of oil in the secondarycircuit. These means include a conduit connecting the oil separator andthe oil tank, which conduit is provided with a solenoid valve controlledby an oil level sensor in the oil tank. This sensor also controlsanother valve in a conduit through which excess oil in the tank iswithdrawn to the compressor inlet. These control equipment makes thecompressor complicated.

SUMMARY OF THE INVENTION

The object of the present invention is to attain a rotary displacementcompressor having a simple and reliable dual liquid circulation system.

In order to achieve the above object, a rotary displacement compressoraccording to the present invention has an inlet channel for introducinglow pressure gas, an outlet channel through which compressed gasescapes, and at least one rotor mounted in bearings and operating in aworking space. The compressor includes a liquid injector for injectingliquid into the working space, a liquid separator provided in the outletchannel for separating liquid from the compressed gas, and a pressureliquid conduit connecting the liquid separator and the liquid injectionport. A bearing lubrication circuit includes a tank, a pump, a supplyconduit for supplying liquid from the pump to the bearings in which therotor is mounted, a withdrawal conduit for withdrawing liquid from thebearings to the tank, and a pump inlet conduit for supplying liquid fromthe tank to the pump. The bearing lubrication circuit further includes aleakage path connecting the tank to a first cavity in the working spaceand a drainage connection connecting the tank to a second cavity in theworking space, the first cavity having a higher pressure than the secondcavity.

By connecting the tank through the drainage connection to the workingspace where a certain intermediate pressure prevails, the oil in thetank will be under that pressure. Through the leakage connection a smallamount of oil will leak from the high pressure side of the compressor tothe tank to secure a sufficient amount of oil in the bearing lubricationcircuit, and through the drainage connection excess of oil in thatcircuit will be returned to the working space. These connections willthus regulate the system so that an appropriate oil volume will bemaintained in the bearing lubrication circuit.

The compressor preferably is a screw compressor. Preferably the firstcavity is at compressor end pressure and the second cavity at anintermediate pressure slightly above inlet pressure. In an advantageousembodiment of the invention, the leakage connection is established bythe clearance around the high pressure shaft journal of the rotors.

DETAILED DESCRIPTION

The invention will be further explained through the following detaileddescription of a preferred embodiment thereof and with reference to theaccompanying drawing which schematically illustrates a compressoraccording to the invention.

The compressor 1 in the FIGURE is a rotary screw compressor having amale rotor 2 meshing with a female rotor (not shown) in a working space3 limited by a barrel section 4, an inlet end section 5 and an outletend section 6. The male rotor has one shaft journal 7 extending throughthe low pressure inlet end section 5 for drive connection with anengine. The other shaft journal 8 extends into the outlet end section 6.Each shaft journal is mounted in bearings 9 and 10, respectively. Thecompressor receives the gas at low pressure via an inlet channel 13through an inlet port 11 and the compressed gas escapes through anoutlet port 12 connected to an outlet channel 14.

The compressor is of the oil injection type, in which oil for cooling,lubrication and sealing purposes is injected through an oil injectionport 15. In the outlet channel 14 there is provided an oil separator 16in which the oil is separated from the compressed gas and recirculatedto the working space 3 via a pressure oil conduit 18 and the injectionport 15, and the oil-.free gas leaves the separator through a deliverychannel 17.

A secondary oil circuit is provided for lubricating the bearings 9, 10at each shaft journal 7, 8. In that circuit a circulation pump 20 pumpsoil through a conduit 21 and branch conduits 22, 23 to the bearings 9,10, from where the oil is drained to an oil tank 26 provided at theoutlet end section 6, the end section itself forming a part of the tank.The drainage from the bearings 9 in the inlet end section 5 isaccomplished through a withdrawal conduit 25 and from the bearings 10 inthe outlet end section 6 directly through the interior of this section.

The clearance between the shaft journal 8 at the outlet end constitutesa leakage path 31 through which oil can leak from the high pressure endof the compressor into the outlet end section 6, i.e. into the oil tank26. And through a drainage connection 32 the tank 26 is in communicationwith the working space of the compressor, which connection ends in theworking space where the pressure is lower than the compressor endpressure, preferably slightly above inlet pressure, which pressure thuswill prevail in the oil tank 26. The same pressure will also prevail inthe inlet end section 5.

The lubrication oil is sucked from the tank 26 by the circulation pump20, which raises the pressure enough for delivering the oil to thebearings 9, 10 via a filter 29, conduit 21 and the branch conduits 22and 23. Due to the relatively low pressure in the bearing lubricationcircuit, the amount of working medium dissolved in the oil will bemoderate and the lubrication ability of the oil will be sufficientlymaintained.

Since oil from the oil injection circuit is allowed to leak from thehigh pressure end of the compressor working space along the shaftjournal 8 into the outlet end section 6 enough oil will be present inthe lubrication circuit. Any excess of oil in that circuit will flowthrough the drainage connection 32 back to the working space, where itis introduced at an early stage of the compression cycle.

In the arrangement described hereinabove, the bearing lubricationcircuit and the oil injection circuit will operate at different pressurelevels, making a relatively low pressure for the bearing lubrication oilpossible. And a minor exchange of oil between the circuits takes place,through which the oil amount in the lubrication circuit is controlled ina simple and reliable way. The oil in the lubrication circuit has to befree from particles to a higher degree than the oil in the injectioncircuit, and is filtered through a high quality filter of fine mesh,whereas the filter 30 in the oil injection circuit can be of a moresimple kind. The high quality filter 29 therefore can be dimensioned totake care of a relatively small amount of particles in comparence whatwould be required with a common system where all the oil would have tobe highly filtered. Due to the lower pressure difference across thisfilter 29 in a system according to the invention, the requirement of thefilter also in this respect will be smaller, allowing a cheaper filterto be used.

In the illustrated embodiment the bearing lubrication circuit is usedalso for supplying oil to the shaft sealing 33 of the driving shaft, towhich it is supplied through a branch conduit 24 and returned throughconduit 35. The compressor is also provided with a thrust balancingpiston 34, to which oil is supplied from the oil separator 16 through apressure oil branch conduit 36, and the oil leakage across the piston 34is drained through a drainage conduit 37 to the oil tank 26 oflubrication system.

We claim:
 1. A rotary displacement compressor having an inlet channelfor introducing low pressure gas, an outlet channel through whichcompressed gas escapes, and at least one rotor mounted in bearings andoperating in a working space, said compressor comprising:a liquidinjection port for injecting liquid into said working space, a liquidseparator provided in said outlet channel for separating liquid from thecompressed gas; a pressure liquid conduit connecting said liquidseparator and said liquid injection port; and a bearing lubricationcircuit including a tank, a pump, a supply conduit for supplying liquidfrom said pump to said bearings in which said rotor is mounted, awithdrawal conduit for withdrawing liquid from said bearings to saidtank, and a pump inlet conduit for supplying liquid from said tank tosaid pump, wherein said bearing lubrication circuit further includes aleakage path connecting said tank to a first cavity in said workingspace and a drainage connection connecting said tank to a second cavityin said working space, said first cavity having a higher pressure thansaid second cavity.
 2. The compressor according to claim 1, wherein saidcompressor is a screw compressor having two meshing screw rotors.
 3. Thecompressor according to claim 2, wherein said first cavity is at acompressor end pressure and said second cavity is at a pressure slightlyabove an inlet pressure.
 4. The compressor according to claim 3, whereinsaid leakage path comprises a clearance around a rotor shaft journal ata high pressure end of the compressor, and wherein said drainageconnection communicates with said tank at a lower level than saidleakage path.
 5. The compressor according to claim 1, wherein saidsupply conduit in said lubrication circuit includes a first filter, andsaid pressure liquid conduit includes a second filter, said first filterbeing capable of separating smaller particles than said second filter.6. The compressor according to claim 1, further comprising a unitconnected to said supply conduit by a branch conduit, said unit beingsupplied with liquid at substantially a same pressure as the liquidsupplied to said bearings in which said rotor is mounted.
 7. Thecompressor according to claim 1, further comprising an additional unitconnected to said pressure liquid conduit by a pressure liquid branchconduit, and connected to said tank by a liquid drainage conduit, saidadditional unit being supplied with liquid at a higher pressure than theliquid supplied to said bearings in which said rotor is mounted.
 8. Thecompressor according to claim 2, wherein said supply conduit in saidlubrication circuit includes a first filter, and said pressure liquidconduit includes a second filter, said first filter being capable ofseparating smaller particles than said second filter.
 9. The compressoraccording to claim 3, wherein said supply conduit in said lubricationcircuit includes a first filter, and said pressure liquid conduitincludes a second filter, said first filter being capable of separatingsmaller particles than said second filter.
 10. The compressor accordingto claim 4, wherein said supply conduit in said lubrication circuitincludes a first filter, and said pressure liquid conduit includes asecond filter, said first filter being capable of separating smallerparticles than said second filter.
 11. The compressor according to claim2, further comprising a unit connected to said supply conduit by abranch conduit, said unit being supplied with liquid at substantially asame pressure as the liquid supplied to said bearings in which saidrotor is mounted.
 12. The compressor according to claim 3, furthercomprising a unit connected to said supply conduit by a branch conduit,said unit being supplied with liquid at substantially a same pressure asthe liquid supplied to said bearings in which said rotor is mounted. 13.The compressor according to claim 4, further comprising a unit connectedto said supply conduit by a branch conduit, said unit being suppliedwith liquid at substantially a same pressure as the liquid supplied tosaid bearings in which said rotor is mounted.
 14. The compressoraccording to claim 2, further comprising an additional unit connected tosaid pressure liquid conduit by a pressure liquid branch conduit, andconnected to said tank by a liquid drainage conduit, said additionalunit being supplied with liquid at a higher pressure than the liquidsupplied to said bearings in which said rotor is mounted.
 15. Thecompressor according to claim 3, further comprising an additional unitconnected to said pressure liquid conduit by a pressure liquid branchconduit, and connected to said tank by a liquid drainage conduit, saidadditional unit being supplied with liquid at a higher pressure than theliquid supplied to said bearings in which said rotor is mounted.
 16. Thecompressor according to claim 4, further comprising an additional unitconnected to said pressure liquid conduit by a pressure liquid branchconduit, and connected to said tank by a liquid drainage conduit, saidadditional unit being supplied with liquid at a higher pressure than theliquid supplied to said bearings in which said rotor is mounted.
 17. Thecompressor according to claim 6, wherein said unit comprises a shaftsealing of a driving shaft of said rotor.
 18. The compressor accordingto claim 13, wherein said unit comprises a shaft sealing of a drivingshaft of said rotor.
 19. The compressor according to claim 7, whereinsaid additional unit comprises a thrust balancing piston.
 20. Thecompressor according to claim 16, wherein said additional unit comprisesa thrust balancing piston.